<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>窗口高度100%</title>
</head>
<style>
  * {
    margin: 0;
    bottom: 0;
  }

  html,
  body,
  .app {
    height: 100%;
  }

  .child {
    height: 100%;
    background-image: linear-gradient(180deg, rgb(250, 60, 2), blue, rgb(6, 201, 6)100%);
  }
</style>

<body>
  <div class="app">
    <div class="child">
    </div>
  </div>
</body>

</html>